RECOVERY OF METAL VALUES FROM SPENT HYDRODESULFURIZATION CATALYSTS BY LIQUID-LIQUID-EXTRACTION

The total dissolution of spent hydrodesulfurization catalysts in sulfuric acid yields an acidic solution containing a certain amount of rare metals like molybdenum, vanadium, cobalt, nickel, and a small amount of iron as well as an appreciable amount of aluminum. For the purpose of recovering the metallic values present in the solution, in the current work, the fundamental extraction characteristics of these metals with some typical commercial acidic organophosphorus compounds such as TR-83, PC-88A, PIA-8, and CYANEX 272 from the sulfuric acid media were investigated, and CYANEX 272 and PIA-8 were found to be suitable for selectively separating and recovering molybdenum and vanadium over the other metals from the specific solution at comparatively low pH. In addition, in order to further recover cobalt and nickel from the raffinate in the presence of a large excess of aluminum, a systematic investigation was conducted on the synergistic extraction behaviors of the mixed solvents consisting of LIX 63, an aliphatic alpha-hydroxyoxime extractant, and a series of acidic extractants such as CYANEX 272, PIA-8, PC-88A, D(2)EHPA, TR-83, and OPEHPA for cobalt, nickel, and aluminum. It was elucidated that all of these mixtures could exhibit a good synergistic effect for extraction of cobalt and nickel and that the lower the pK(a) of the acidic extractants, the greater the synergism. Contrarily, the extraction of aluminum, was considerably depressed by LIX 63. Using the difference in synergistic extraction performance between these metals, the effective extraction of cobalt and nickel away from aluminum was achieved from the sulfate solution at relatively low pH with the mixtures. In particular, the mixtures of LM 63 in combination with CYANEX 272 or PIA-8 appeared to be the most feasible and economic from a practical point of view yielding acceptable separation efficiency for cobalt and nickel over aluminum and a low acidity requirement for stripping cobalt and nickel as well as negligible degradation of LIX 63. A full recovery processing route is proposed based on the experimental results.